The on-again off-again couple were spotted in Sin City early Sunday morning. With a group of friends, the duo spent their night out at Spearmint Rhino ... a strip club! "They were together in a group,” an onlooker tells PEOPLE. "As for whether they got dances, they were in a strip club ... you do the math.”  The group had so much fun, they stayed until 5 a.m.  Earlier in the evening, Perry and Mayer checked into the Wynn Hotel together. People Magazine